A stud welder comprises a power source and a gun or head which includes a collet to carry a stud to be welded to a workpiece. Welding is performed by first contacting the stud and the workpiece, and passing a relatively low current, called a pilot current, through the stud and workpiece. While this pilot current is flowing, the stud is lifted or moved away from the workpiece by a relatively short predetermined distance by means of a lift solenoid. This creates a pilot arc. Then, a much larger current, called the main arc current, is sent through the arc which was drawn when the stud was raised with respect to the workpiece. The main current is applied for a predetermined time sufficient to cause the stud and the workpiece to melt. Then, after a predetermined time after the main arc is stopped, the stud is urged into the workpiece to thereby weld the stud to the workpiece.
The time between the cessation of the main arc until actual contact of the stud on the workpiece is critical. This time is called "plunge time". If the plunge time is too long, the stud and/or the workpiece will have cooled to the point where no weld will occur. If the plunge time is too short, then the molten metal will splash, and a poor weld or no weld will result.